Technical Field
The invention relates to a method and to an apparatus for read measurement of a plurality of resistive memory cells having a plurality of programmable levels.
Description of the Related Art
A prominent example for resistive memory cells having a plurality of programmable levels is Resistive Random Access Memory (RRAM), particular Phase Change Memory (PCM). PCM is a non-volatile solid-state memory technology that exploits the reversible, thermally-assisted switching of specific chalcogenides between certain states of different electrical conductivity.
PCM is a promising and advanced emerging non-volatile memory technology mainly due to its excellent features including low latency, high endurance, long retention and high scalability. PCM may be considered a prime candidate for Flash replacement, embedded/hybrid memory and storage-class memory. Key requirements for competitiveness of PCM technology may be multi-level cell functionality, in particular for low cost per bit, and high-speed read/write operations, in particular for high bandwidth. Multilevel functionality, i.e. multiple bits per PCM cell, may be a way to increase storage capacity and thereby to reduce cost.
Multi-level PCM is based on storing multiple resistance levels between a lowest (SET) and a highest (RESET) resistance value. Multiple resistance levels or levels correspond to partial-amorphous and partial-crystalline phase distributions of the PCM cell. Phase transformation, i.e. memory programming, may be enabled by Joule heating. In this regard, Joule heating may be controlled by a programming current or voltage pulse. Storing multiple resistance levels in a PCM cell is a challenging task.
For example, one reference describes that the multiple states or levels in a PCM cell are created by varying the programming power, thus creating different crystalline and amorphous fractions within the cell. Further according to another reference, in metal-oxide resistive memory devices, multiple states may correspond to variations in the gap between conductive oxygen-vacancy filaments and the electrodes.
However, in many of such devices, temporal drift may pose a significant challenge to the realization of reliable multi-level functionality. The current-voltage characteristics associated with the various programmed levels exhibit temporal changes. This is usually attributed to the deviations in the band-gap and/or defect density of the memory materials over time. Low-field resistance that is widely used to differentiate between the various programmed states is particularly susceptible to drift.
In this regard, FIGS. 1 to 4 show an example illustrating drift in PCM cell having four levels (states). In this regard, FIG. 1 shows a read voltage 101 which is constant over time. FIG. 2 illustrates the resistance drift 201-204 of the four levels. The current drift is inverse to the resistance drift and shown by corresponding graphs 301-303 of FIG. 3. Further, FIG. 4 shows the drift coefficient for the resistance (curve 401) and for the current (curve 402). Recapitulating FIGS. 1 to 4, the low-field electrical resistance is particularly sensitive to the variation in activation energy applied by the current. When the cells are read with a constant read voltage, both the resistance R and the current I will drift with a drift coefficient having the same magnitude, but opposite polarity (see FIG. 4).
To counter drift, several techniques are known. One of the known techniques is the use of reference cells programmed to multiple resistance levels. These references may be programmed together with the programming of regular data cells or memory cells. By monitoring and characterizing the drift behaviour of each of the programmed levels, the drift in the regular memory cells may be partially accounted for, for example by altering the threshold levels for detection.
Another known technique is the use of so-called non-resistance-based cell-state-metrics as in the case of PCM cells (it is shown that the sub-threshold slope of the current-voltage curve corresponding to each programmed level is indicative of the amorphous/crystalline ratio and is not susceptible to drift. Several read-out schemes for estimating the slope are known.
In one reference, several coding strategies are proposed to address drift.
Accordingly, it is an aspect of the present invention to improve the read measurement of a plurality of resistive memory cells.